Support a local storage for developer

[La0]

The solution is dependant on a Google Cloud Storage account.
A contributor cannot run the backend nor the full bot without such an account, thus limiting contributions.

There does not seem to exist any "minio for GCS" in order to run a local fake GCS server.
As we do not use a lot of API calls for that service, it should be doable to support a local storage instead of GCS.

We need to be able to:

• list blobs
• check if a specific blob is available
• read a blob
• write a blob